Since gear box manufacturing costs are primarily governed by weight and since this in turn influences nacelle and tower weights, then significant savings can be made in a complete installation.
This invention therefore, exploits the fact that an epicyclic gear with a low basic annulus to sun ratio can accommodate a greater number of planet wheels than a gear with a high ratio. Furthermore, the lower the ratio, the more efficient the gear becomes when used in a planetary configuration with a stationary annulus because the lower relative speed of the sunwheel with respect to the planet carrier, ensures that a greater proportion of the throughput power is transmitted by a direct coupling effect. Since a planet carrier has a torque equal numerically to the sum of the sun and annulus torques, then to minimise volume, it should always be used as the high torque member in a transmission. However, to realise the full benefit of using more than three planets it is essential to have an effective means of sharing the load equally between them e.g. the flexible pin planet spindle which enables the maximum number to be used subject only to the clearance between adjacent planet tip diameters. The key feature of this invention is that total input torque is arbitrarily divided between two low ratio primary planetary trains that transmit power along two parallel paths. These low ratios stem from the use of a secondary differential train to re-combine the separated power flows into a single output. The resultant overall speed increasing ratio is effectively doubled and for a given output speed, the primary ratios can be halved. This enables more planets to be used so that there is a significant reduction in collective weight and component sizes.